In Cambodian Architecture, the period of Angkor is the period in the history of the Khmer Empire from approximately the later half of the 8th century AD to the first half of the 15th century CE.

In any study of Angkorian architecture, the emphasis is necessarily on religious architecture, since all the remaining Angkorian buildings are religious in nature. During the period of Angkor, only temples and other religious buildings were constructed of stone. Non-religious buildings such as dwellings were constructed of perishable materials such as wood, and so have not survived.

The religious architecture of Angkor has characteristic structures, elements, and motifs, which are identified in the glossary below. Since a number of different architectural styles succeeded one another during the Angkorean period, not all of these features were equally in evidence throughout the period. Indeed, scholars have referred to the presence or absence of such features as one source of evidence for dating the remains.

Periodization
Many temples had been built before Cambodia became a powerful Kingdom of Khmer Empire which dominated most of the Indochina region. At that time, Cambodia was known as Chenla kingdom, the predecessor state of Khmer empire. There are three pre-Angkorean architectural styles :

Sambor Prei Kuk style (610-650 AD): Sambor Prei Kuk also known as Isanapura where was the capital of Chenla Kingdom. Temples of Sambor Prei Kuk were built in round shape, plain colonettes with capitals that include a bulb.
Prei Khmeng style (635-700 AD): Structures reveal masterpieces of sculpture but architecture scarce. Colonettes are larger than previous style. Buildings were more heavily decorated but they had general decline of standards.
Kompong Preah style (700-800 AD): Temples with more decorative rings on colonettes which remain cylindrical. Brick constructions were being continued.

Scholars have worked to develop a periodization of Angkorean architectural styles. The following periods and styles may be distinguished. Each is named for a particular temple regarded as paradigmatic for the style.

Kulen style (825-875 AD): Continuation of pre-Angkorean style but it was a period of innovation and borrowing such as from Cham temples. Tower is mainly square and relatively high as well as brick with laterite walls and stone door surrounds but square and octagonal colonettes begin to appear.
Preah Ko style (877-886 AD): Hariharalaya was the first capital city of the Khmer empire located in the area of Angkor; its ruins are in the area now called Roluos some fifteen kilometers southeast of the modern city of Siem Reap. The earliest surviving temple of Hariharalaya is Preah Ko; the others are Bakong and Lolei. The temples of the Preah Ko style are known for their small brick towers and for the great beauty and delicacy of their lintels.
Bakheng Style (889-923): Bakheng was the first temple mountain constructed in the area of Angkor proper north of Siem Reap. It was the state temple of King Yasovarman, who built his capital of Yasodharapura around it. Located on a hill (phnom), it is currently one of the most endangered of the monuments, having become a favorite perch for tourists eager to witness a glorious sundown at Angkor.
Koh Ker Style (921-944): During the reign of King Jayavarman IV, capital of Khmer empire was removed from Angkor region through the north which is called Koh Ker. The architectural style of temples in Koh Ker, scale of buildings diminishes toward center. Brick still main material but sandstone also used.
Pre Rup Style (944-968): Under King Rajendravarman, the Angkorian Khmer built the temples of Pre Rup, East Mebon and Phimeanakas. Their common style is named after the state temple mountain of Pre Rup.
Banteay Srei Style (967-1000): Banteay Srei is the only major Angkorian temple constructed not by a monarch, but by a courtier. It is known for its small scale and the extreme refinement of its decorative carvings, including several famous narrative bas-reliefs dealing with scenes from Indian mythology.
Khleang Style (968-1010): The Khleang temples, first use of galleries. Cruciform gopuras. Octagonal colonettes. Restrained decorative carving. A few temples that were built in this style are Ta Keo, Phimeanakas.
Baphuon Style (1050–1080): Baphuon, the massive temple mountain of King Udayadityavarman II was apparently the temple that most impressed the Chinese traveller Zhou Daguan, who visited Angkor toward the end of the 13th century. Its unique relief carvings have a naive dynamic quality that contrast with the rigidity of the figures typical of some other periods. As of 2008, Baphuon is under restoration and cannot currently be appreciated in its full magnificence.
Classical or Angkor Wat Style (1080–1175): Angkor Wat, the temple and perhaps the mausoleum of King Suryavarman II, is the greatest of the Angkorian temples and defines what has come to be known as the classical style of Angkorian architecture. Other temples in this style are Banteay Samre and Thommanon in the area of Angkor, and Phimai in modern Thailand.
Bayon Style (1181–1243): In the final quarter of the 12th century, King Jayavarman VII freed the country of Angkor from occupation by an invasionary force from Champa. Thereafter, he began a massive program of monumental construction, paradigmatic for which was the state temple called the Bayon. The king’s other foundations participated in the style of the Bayon, and included Ta Prohm, Preah Khan, Angkor Thom, and Banteay Chmar. Though grandiose in plan and elaborately decorated, the temples exhibit a hurriedness of construction that contrasts with the perfection of Angkor Wat.
Post Bayon Style (1243–1431): Following the period of frantic construction under Jayavarman VII, Angkorian architecture entered the period of its decline. The 13th century Terrace of the Leper King is known for its dynamic relief sculptures of demon kings, dancers, and nāgas.

Materials

Angkorian builders used brick, sandstone, laterite and wood as their materials. The ruins that remain are of brick, sandstone and laterite, the wood elements having been lost to decay and other destructive processes.

Brick
The earliest Angkorian temples were made mainly of brick. Good examples are the temple towers of Preah Ko, Lolei and Bakong at Hariharalaya. Decorations were usually carved into a stucco applied to the brick, rather than into the brick itself.

Angkor’s neighbor state of Champa was also the home to numerous brick temples that are similar in style to those of Angkor. The most extensive ruins are at Mỹ Sơn in Vietnam. A Cham story tells of the time that the two countries settled an armed conflict by means of a tower-building contest proposed by the Cham King Po Klaung Garai. While the Khmer built a standard brick tower, Po Klaung Garai directed his people to build an impressive replica of paper and wood. In the end, the Cham replica was more impressive than the real brick tower of the Khmer, and the Cham won the contest.

Sandstone
The only stone used by Angkorian builders was sandstone, obtained from the Kulen mountains. Since its obtainment was considerably more expensive than that of brick, sandstone only gradually came into use, and at first was used for particular elements such as door frames. The 10th-century temple of Ta Keo is the first Angkorian temple to be constructed more or less entirely from Sandstone.

Laterite
Angkorian builders used laterite, a clay that is soft when taken from the ground but that hardens when exposed to the sun, for foundations and other hidden parts of buildings. Because the surface of laterite is uneven, it was not suitable for decorative carvings, unless first dressed with stucco. Laterite was more commonly used in the Khmer provinces than at Angkor itself.

Structures
Central sanctuary
The central sanctuary of an Angkorian temple was home to the temple’s primary deity, the one to whom the site was dedicated: typically Shiva or Vishnu in the case of a Hindu temple, Buddha or a bodhisattva in the case of a Buddhist temple. The deity was represented by a statue (or in the case of Shiva, most commonly by a linga). Since the temple was not considered a place of worship for use by the population at large, but rather a home for the deity, the sanctuary needed only to be large enough to hold the statue or linga; it was never more than a few metres across. Its importance was instead conveyed by the height of the tower (prasat) rising above it, by its location at the centre of the temple, and by the greater decoration on its walls. Symbolically, the sanctuary represented Mount Meru, the legendary home of the Hindu gods.

Prang
The prang is the tall finger-like spire, usually richly carved, common to much Khmer religious architecture.

Enclosure
Khmer temples were typically enclosed by a concentric series of walls, with the central sanctuary in the middle; this arrangement represented the mountain ranges surrounding Mount Meru, the mythical home of the gods. Enclosures are the spaces between these walls, and between the innermost wall and the temple itself. By modern convention, enclosures are numbered from the centre outwards. The walls defining the enclosures of Khmer temples are frequently lined by galleries, while passage through the walls is by way of gopuras located at the cardinal points.

Gallery
A gallery is a passageway running along the wall of an enclosure or along the axis of a temple, often open to one or both sides. Historically, the form of the gallery evolved during the 10th century from the increasingly long hallways which had earlier been used to surround the central sanctuary of a temple. During the period of Angkor Wat in the first half of the 12th century, additional half galleries on one side were introduced to buttress the structure of the temple.

Gopura
A gopura is an entrance building. At Angkor, passage through the enclosure walls surrounding a temple compound is frequently accomplished by means of an impressive gopura, rather than just an aperture in the wall or a doorway. Enclosures surrounding a temple are often constructed with a gopura at each of the four cardinal points. In plan, gopuras are usually cross-shaped and elongated along the axis of the enclosure wall; if the wall is constructed with an accompanying gallery, the gallery is sometimes connected to the arms of the gopura. Many Angkorian gopuras have a tower at the centre of the cross. The lintels and pediments are often decorated, and guardian figures (dvarapalas) are often placed or carved on either side of the doorways.

Hall of Dancers
A Hall of Dancers is a structure of a type found in certain late 12th-century temples constructed under King Jayavarman VII: Ta Prohm, Preah Khan, Banteay Kdei and Banteay Chhmar. It is a rectangular building elongated along the temple’s east axis and divided into four courtyards by galleries. Formerly it had a roof made of perishable materials; now only the stone walls remain. The pillars of the galleries are decorated with carved designs of dancing apsaras; hence scholars have suggested that the hall itself may have been used for dancing.

House of Fire
House of Fire, or Dharmasala, is the name given to a type of building found only in temples constructed during the reign of late 12th-century monarch Jayavarman VII: Preah Khan, Ta Prohm and Banteay Chhmar. A House of Fire has thick walls, a tower at the west end and south-facing windows.

Scholars theorize that the House of Fire functioned as a “rest house with fire” for travellers. An inscription at Preah Khan tells of 121 such rest houses lining the highways into Angkor. The Chinese traveller Zhou Daguan expressed his admiration for these rest houses when he visited Angkor in 1296 AD. Another theory is that the House of Fire had a religious function as the repository the sacred flame used in sacred ceremonies.

Library
Structures conventionally known as “libraries” are a common feature of Khmer temple architecture, but their true purpose remains unknown. Most likely they functioned broadly as religious shrines rather than strictly as repositories of manuscripts. Freestanding buildings, they were normally placed in pairs on either side of the entrance to an enclosure, opening to the west.

Srah and baray
Srahs and barays were reservoirs, generally created by excavation and embankment, respectively. It is not clear whether the significance of these reservoirs was religious, agricultural, or a combination of the two.

The two largest reservoirs at Angkor were the West Baray and the East Baray located on either side of Angkor Thom. The East Baray is now dry. The West Mebon is an 11th-century temple standing at the center of the West Baray and the East Mebon is a 10th-century temple standing at the center of the East Baray.

The baray associated with Preah Khan is the Jayataka, in the middle of which stands the 12th-century temple of Neak Pean. Scholars have speculated that the Jayataka represents the Himalayan lake of Anavatapta, known for its miraculous healing powers.

Temple mountain
The dominant scheme for the construction of state temples in the Angkorian period was that of the Temple Mountain, an architectural representation of Mount Meru, the home of the gods in Hinduism. The style was influenced by Indian temple architecture. Enclosures represented the mountain chains surrounding Mount Meru, while a moat represented the ocean. The temple itself took shape as a pyramid of several levels, and the home of the gods was represented by the elevated sanctuary at the center of the temple.

The first great temple mountain was the Bakong, a five-level pyramid dedicated in 881 by King Indravarman I. The structure of Bakong took shape of stepped pyramid, popularly identified as temple mountain of early Khmer temple architecture. The striking similarity of the Bakong and Borobudur in Java, going into architectural details such as the gateways and stairs to the upper terraces, strongly suggests that Borobudur might served as the prototype of Bakong. There must have been exchanges of travelers, if not mission, between Khmer kingdom and the Sailendras in Java. Transmitting to Cambodia not only ideas, but also technical and architectural details of Borobudur, including arched gateways in corbelling method.

Other Khmer temple mountains include Baphuon, Pre Rup, Ta Keo, Koh Ker, the Phimeanakas, and most notably the Phnom Bakheng at Angkor.:103,119

According to Charles Higham, “A temple was built for the worship of the ruler, whose essence, if a Saivite, was embodied in a linga… housed in the central sanctuary which served as a temple-mausoleum for the ruler after his death…these central temples also contained shrines dedicated to the royal ancestors and thus became centres of ancestor worship.”:351

Elements
Bas-relief
Bas-reliefs are individual figures, groups of figures, or entire scenes cut into stone walls, not as drawings but as sculpted images projecting from a background. Sculpture in bas-relief is distinguished from sculpture in haut-relief, in that the latter projects farther from the background, in some cases almost detaching itself from it. The Angkorian Khmer preferred to work in bas-relief, while their neighbors the Cham were partial to haut-relief.

Narrative bas-reliefs are bas-reliefs depicting stories from mythology or history. Until about the 11th century AD, the Angkorian Khmer confined their narrative bas-reliefs to the space on the tympana above doorways. The most famous early narrative bas-reliefs are those on the tympana at the 10th-century temple of Banteay Srei, depicting scenes from Hindu mythology as well as scenes from the great works of Indian literature, the Ramayana and the Mahabharata. By the 12th century, however, the Angkorian artists were covering entire walls with narrative scenes in bas-relief. At Angkor Wat, the external gallery wall is covered with some 12,000 or 13,000 square meters of such scenes, some of them historical, some mythological. Similarly, the outer gallery at the Bayon contains extensive bas-reliefs documenting the everyday life of the medieval Khmer as well as historical events from the reign of King Jayavarman VII.

The following is a listing of the motifs illustrated in some of the more famous Angkorian narrative bas-reliefs:

bas-reliefs in the tympana at Banteay Srei (10th century)
the duel of the monkey princes Vali and Sugriva, and the intervention of the human hero Rama on behalf of the latter
the duel of Bhima and Duryodhana at the Battle of Kurukshetra
the Rakshasa king Ravana shaking Mount Kailasa, upon which sit Shiva and his shakti
Kama firing an arrow at Shiva as the latter sits on Mount Kailasa
the burning of Khandava Forest by Agni and Indra’s attempt to extinguish the flames

bas-reliefs on the walls of the outer gallery at Angkor Wat (mid-12th century)
the Battle of Lanka between the Rakshasas and the vanaras or monkeys
the court and procession of King Suryavarman II, the builder of Angkor Wat
the Battle of Kurukshetra between Pandavas and Kauravas
the judgment of Yama and the tortures of Hell
the Churning of the Ocean of Milk
a battle between devas and asuras
a battle between Vishnu and a force of asuras
the conflict between Krishna and the asura Bana
the story of the monkey princes Vali and Sugriva

bas-reliefs on the walls of the outer and inner galleries at the Bayon (late 12th century)
battles on land and sea between Khmer and Cham troops
scenes from the everyday life of Angkor
civil strife among the Khmer
the legend of the Leper King
the worship of Shiva
groups of dancing apsaras

Blind door and window
Angkorean shrines frequently opened in only one direction, typically to the East. The other three sides featured fake or blind doors to maintain symmetry. Blind windows were often used along otherwise blank walls.

Colonette
Colonettes were narrow decorative columns that served as supports for the beams and lintels above doorways or windows. Depending on the period, they were round, rectangular, or octagonal in shape. Colonettes were often circled with molded rings and decorated with carved leaves.

Corbelling
Angkorian engineers tended to use the corbel arch in order to construct rooms, passageways and openings in buildings. A corbel arch is constructed by adding layers of stones to the walls on either side of an opening, with each successive layer projecting further towards the centre than the one supporting it from below, until the two sides meet in the middle. The corbel arch is structurally weaker than the true arch. The use of corbelling prevented the Angkorian engineers from constructing large openings or spaces in buildings roofed with stone, and made such buildings particularly prone to collapse once they were no longer maintained. These difficulties did not, of course, exist for buildings constructed with stone walls surmounted by a light wooden roof. The problem of preventing the collapse of corbelled structures at Angkor remains a serious one for modern conservation.

Lintel, pediment, and tympanum
A lintel is a horizontal beam connecting two vertical columns between which runs a door or passageway. Because the Angkorean Khmer lacked the ability to construct a true arch, they constructed their passageways using lintels or corbelling. A pediment is a roughly triangular structure above a lintel. A tympanum is the decorated surface of a pediment.

The styles employed by Angkorean artists in the decoration of lintels evolved over time, as a result, the study of lintels has proven a useful guide to the dating of temples. Some scholars have endeavored to develop a periodization of lintel styles. The most beautiful Angkorean lintels are thought to be those of the Preah Ko style from the late 9th century.

Common motifs in the decoration of lintels include the kala, the nāga and the makara, as well as various forms of vegetation. Also frequently depicted are the Hindu gods associated with the four cardinal directions, with the identity of the god depicted on a given lintel or pediment depending on the direction faced by that element. Indra, the god of the sky, is associated with East; Yama, the god of judgment and Hell, with South; Varuna, the god of the ocean, with West; and Kubera, god of wealth, with North.

List of Khmer lintel styles

Sambor Prei Kuk style : Inward-facing makaras with tapering bodies. Four arches joined by three medallions, the central once carved with Indra. Small figure on each makara. A variation is with figures replacing the makaras and a scene with figures below the arch.
Prei Khmeng style : Continuation of Sambor Prei Kuk but makaras disappear, being replaced by incurving ends and figures. Arches more rectilinear. Large figures sometimes at each end. A variation is a central scene below the arch, usually Vishnu Reclining.
Kompong Preah style : High quality carving. Arches replaced by a garland of vegetation (like a wreath) more or less segmented. Medallions disappear, central one sometimes replaced by a knot of leaves. Leafy pendants spray out above and below garland.
Kulen style : Great diversity, with influences from Champa and Java, including the kala and outward-facing makaras.
Preah Ko style : Some of the most beautiful of all Khmer lintels, rich, will-carved and imaginative. Kala in center, issuing garland on either side. Distinct loops of vegetation curl down from garland. Outward-facing makaras sometimes appear at the ends. Vishnu on Garuda common.
Bakheng style : Continuation of Preah Ko but less fanciful and tiny figures disappear. Loop of vegetation below the naga form tight circular coils. Garland begins to dip in the center.
Koh Ker style : Center occupied by a prominent scene, taking up almost the entire height of the lintel. Usually no lower border. Dress of figures shows a curved line to the sampot tucked in below waist.
Pre Rup style : Tendency to copy earlier style, especially Preah Ko and Bakheng. Central figures. Re-appearance of lower border.
Banteay Srei style : Increase in complexity and detail. Garland sometimes makes pronounced loop on either side with kala at top of each loop. Central figure.
Khleang style : Less ornate than those of Banteay Srei. Central kala with triangular tongue, its hands holding the garland which is bent at the center. Kala sometimes surmounted by a divinity. Loops of garland on either side divided by flora stalk and pendant. Vigorous treatment of vegetation.
Baphuon style : The central kala surmounted by divinity, usually riding a steed or a Vishnu scene, typically from the life of Krishna. Loops of garland no longer cut. Another type is a scene with many figures and little vegetation.
Angkor Wat style : Centered, framed and linked by garlands. A second type is a narrative scene filled with figures. When nagas appear, they curls are tight and prominent. Dress mirrors that of devatas and apsaras in bas-reliefs. No empty spaces.
Bayon style : Most figures disappear, usually only a kala at the bottom of the lintel surmounted by small figure. Mainly Buddhist motifs. In the middle of the period the garland is cut into four parts, while later a series of whorls of foliage replace the four divisions.

Stairs
Angkorean stairs are notoriously steep. Frequently, the length of the riser exceeds that of the tread, producing an angle of ascent somewhere between 45 and 70 degrees. The reasons for this peculiarity appear to be both religious and monumental. From the religious perspective, a steep stairway can be interpreted as a “stairway to heaven,” the realm of the gods. “From the monumental point of view,” according to Angkor-scholar Maurice Glaize, “the advantage is clear – the square of the base not having to spread in surface area, the entire building rises to its zenith with a particular thrust.”

Source From Wikipedia